PREFACE. ix 



plates were lithographed, from i to Ixxx, for illustrating the material then on 

 hand, and the additional plates can only come in after the latter number. This 

 want of conformity, however, affects only the plates, the text presenting the 

 genera and species in consecutive and systematic order, with cross-references 

 to the newly introduced plates. 



The subdivision among those forms which have usually been referred to 

 Aviculopecten was found to be imperatively necessary for any strict classifica- 

 tion, and the other subdivisions among generic forms which have indiscrimi- 

 nately been referred to Avicula, Pterinea, Pteronites, etc., have been deemed 

 equally important. In these subdivisions, while the essential internal characters 

 have been regarded as of primary importance, the author, without violating 

 this rule, has endeavored to make such an arrangement of the species that the 

 student may determine the generic relations from the general form and exterior 

 markings alone. Since the fossil Lamellibranchiata are commonly found in an 

 imperfect condition, with the interior surface remaining attached to the matrix, 

 and only in rare examples, or in very favorable conditions, revealing the interior 

 structure, it becomes important to have some means of identification other than 

 that furnished by the hinge characters which are so rarely accessible. However, 

 notwithstanding the general arrangement according to external features, the 

 hinge and the interior structure have by no means been undervalued or neglected, 

 as is shown by numerous illustrations of these parts, which it is believed give 

 an amount of information not liefore published in any work upon this class of 

 fossils. 



The synopsis of genera here presented is given without comment or com- 

 parison. In the succeeding volume the author proposes to give a resume of all 

 the genera described, and in the same connection a comparison with genera 

 described in other publications with some notice of the bibliography of this 

 class of fossils, so far as relates to the palaeozoic forms. In that notice the 

 reasons for the generic subdivisions proposed in the present volume will be 

 given in full. 



In the final revision of the genera and species and in the preparation of the 



B 



